Mr. Fixit
FYI the capacity reference you'll find in the l Japanese model names & many European models are base on a 500mm load center (~20") vs the US std. of 24" (600mm). So the capacity reference is based on the Japanese market standard of 500 mm. While your calculation of 32 = 7048 is accurate but only for 20" l.c.

TCM Girl - there may be other differences besides counterweight when converting a 6000 lb to a 6500 lb. model like a different channel over lap, a larger hydraulic pump, different front & rear tires, etc. When it comes to rearward visibility. There are design stds. all manufacturers must comply with with (including with an LPG tank installed). Even if the difference between a 6000 to a 6500 lb unit MIGHT be 500 lbs. all the weight is not necessarily "stacked" on top of the counterweight (unless a dealer did it on his own). The actual counterweight increase in weight is actually less than 500 lb as the at the center of the rear tires is much further away (aka the trucks wheel base) from the center of the drive axle (fulcrum) than from the center of the drive axle to the load center by ABOUT 2 to 1. So the extra counter weight needed is more or less 250 - 300 lbs. - just a SWAG - but an intelligent one - was a Product Manager for many years..